The present invention relates to catheters, and more particularly, to catheter guidewires and core and safety wires for use therewith.
Current techniques of introducing a catheter into the vascular system of a patient include the followng steps: insertion of a sharp cannula through the skin and into the vascular system, insertion of a spring guidewire through the cannula and into the vascular system, and removal of the cannula from the patient's body and insertion of the catheter into the body by sliding said catheter over the guidewire. The guidewire is then withdrawn, and the catheter is ready for further positioning and use.
It should be evident that the guidewire must be flexible and yet strong. It must be flexible enough to negotiate the desired tortuous path of the vascular system and do no damage with its leading tip portion and yet be strong enough to resist doubling back, kinking or breaking during the insertion and retraction procedures. It is accordingly desirable that the guidewire have a flexible and yet guidable distal tip, and a relatively stiff, strong elongated body portion. In addition to the foregoing, the guidewire should have an ultra-smooth outer surface.
A guidewire, sometimes referred to as a spring guide, is constructed of a finely wound spring with one or more wires running longitudinally within the spring's central lumen. More detailed structure and function will be explained later.
An ultra-smooth outer guidewire surface is desirable because rough surfaces can traumatize the patient's tissues during movement of the guidewire. In addition, a smooth guidewire surface facilitates cleaning, thus minimizing the possibility of introducing potential toxic or pyrogenic material to the patient's system.
While to the naked eye, the outer surface of a typical guidewire will appear smooth, microscopic surface irregularities are oftentimes present.
Any biologically foreign material introduced into the bloodstream may initiate blood clots (thrombosis). Such thrombogenicity is an undesirable side effect of known angiographic guidewires, increasing the possibility of generating blood clot emboli in the vascular system with the angiographic procedure. Blood clot generation (thrombosis) may be partially a function of the quantity of foreign material surface area. It is well known that surface roughness may multiply the actual exposed surface area of a material many times over a equivalent quantity of smooth area. Thus surface smoothness in itself is a factor in thromboresistance.
An existing approach to a blood clot resistant guidewire surface has been the chemical bonding of an anticoagulant material to the guidewire surface. The present invention offers a novel alternative to a chemically bonded anticoagulant.
One known type of guidewire is developed from a coiled flatwire which forms the outer casing. As used here, a flatwire has an approximately rectangular cross section. While the flatwire has several properties which are favorable to its use as a guidewire outer casing such as increased strength and resistance to fracture, it also has drawbacks. Due to internal forces which are developed during the required winding, or coiling operation, the flatwire tends to take a concave shape across its periphery, becoming somewhat sharp at its edges. Some attempts have been made to overcome this drawback of concavity with edging by coating or grinding the wires after winding. However, neither coating nor grinding has been found satisfactory because coatings often crack during guidewire flexing and grinding still leaves microscopic surface irregularities.
Structural integrity with flexibility is another requisite for a successful guidewire. A broken guidewire, with the possibility of leaving debris in the patient's body cannot be tolerated. A commonly employed precaution against leaving a broken guidewire tip in the vascular system of a patient, is the provision of a thin wire termed a safety wire inside the wound outer guidewire casing. The safety wire is customarily soldered both to the proximal and distal ends of the guidewire to enable the removal of a broken distal fragment should a break occur in the outer spring or guidewire casing. Then, to provide some degree of rigidity to the guidewire body, a core wire is frequently positioned coextensive with the safety wire, inside the outer casing of the guidewire, and connected to the proximal end but ending freely several centimeters short of the guidewire distal tip. Such a structure as this has resulted in the problem that there is a semi-abrupt change from rigidity to flexibility at the body tip junction. Such a change of flexibility may produce a breakpoint when the guidewire is subjected to repeated flexion.
One might postulate that by tapering the core wire down to a thin flexible tip portion that the breakpoint could be eliminated. However, if this portion is free and not connected distally, such a thin free tip portion could protrude from the spring casing with excessive flexion and rough handling of the guidewire. An interesting alternative would be to taper down the core wire to achieve the desired flexibility and connect the tip to the end of the guidewire thus combining the functions of the safety wire and core wire into one safety-core wire. In practice one manufacturer has done essentially this by grinding a taper on the core wire and soldering a braid to the core wire tip which in turn is connected to the tip of the guidewire. Another approach has been to grind a taper followed by a reduced diameter tip portion which is subsequently rolled to a flattened cross-sectional configuration. The first alternative is labor intensive and adds an extra solder connection. The second method runs a risk of weakening or damaging the safety-core wire from the secondary operations of grinding and rolling.
The guidewire of the present invention overcomes the above drawbacks.